The present invention relates to a screwing apparatus for screwing parts to each other by means of screws in an assembling process.
There are demands for the development of a screwing apparatus, installed on robots or provided in assembling processes, which can be automatically operated by a rigid quality control procedure. In particular, in the control of the torque in a screwing operation, a standard fastening torque is provided by the industrial standard, depending on the diameter of a screw and the material of the parts to be fastened. In an assembling process, it is necessary to examine whether or not parts have been screw-fastened at an appropriate fastening torque. But the number of portions to be screw-fastened is so great that it is,difficult to inspect all of them. Therefore, sampling inspections are performed to check whether it is possible to tighten screws by means of a torque wrench or a torque driver. In fastening parts with screws at an appropriate torque, voltage-driving type drivers or current-driving type drivers are utilized to control the rotational force of a bit installed on a motor so as to fasten parts with screws at an appropriate torque.
Referring to FIG. 5, the construction of a conventional screwing apparatus is described below. The screwing apparatus incorporating a brushless motor, is provided with a speed reducer 2 at the bottom thereof. The rotational speed of the brushless motor is reduced by the reducer 2 and the reduced speed thereof is transmitted to a bit 3 so as to rotate a screw 10 (not shown) installed at the lower end of the bit 3. The brushless motor is connected to a current-driving type driver 6 connected to a central processing unit (CPU) 4 via a D/A converter 5. Based on the torque set by the CPU 4, the current-driving type driver 6 drives the brushless motor by controlling the amplitude of the electric current.
The operation of the screwing apparatus having the above-described construction is described below with reference to FIG. 6. The screwing process comprises a screwing region (a) between the start of a screwing operation and a seating state (A) in which a seating surface 7a of a screw head 7 is brought into contact with a workpiece 8; and a fastening region (b) between the seating state (A) and the state in which the screw 10 is fastened at the target fastening torque (B). In a screw-sucking operation to be performed in the screwing region (a), the CPU 4 issues an instruction of a screw-sucking torque value PW1 which is a torque limit set in a low-speed rotation of the brushless motor between the state in which the screw 10 is set on a catcher 9 at a predetermined position thereof and the state in which the screw 10 is sucked by a sucking pipe 11 moved downward to cover the screw head 7 set on the catcher 9. Normally, the screw-sucking torque value PW1 is set to about 2 Kgfcm. A screwing torque value PW2 is a torque limit set in a high-speed rotation of the brushless motor in a screwing operation. The screwing torque value PW2 is set to be greater than the target fastening torque (B) in view of the kind of the screw 10 and the workpiece 8 and the screwing state so as to prevent the generation of a motor locking error in the screwing operation. A seating torque value PW3 is used between the state in which the screw 10 is positioned at a predetermined height 1 mm above the surface of the workpiece 8 and the state in which the screw 10 is seated on the workpiece 8. That is, the seating torque value PW3 is a torque limit at the time when the seating of the screw 34 on the workpiece 32 is detected. Normally, the seating torque value PW3 is set to be equal to a torque, predetermined in the CPU 4, for deciding whether or not the screw 10 has been seated on the workpiece 8. The seating state of the screw 10 is considered in setting the seating torque value PW3. Finally, at a reversing torque value PW4, a reversing brake is operated to seat the screw 10 on the workpiece 8.
At the start in fastening the screw 10 in the fastening region (b), a fastening-start torque value PW5 is used as a torque to smoothly fasten the screw 10 seated on the workpiece 8. Normally, the fastening-start torque value PW5 is set to about half the seating torque value PW3. The rate (Kgfcm) of increase in torque from the fastening-start torque value PW5 until the target fastening torque (B) is set. For example, supposing that the increase rate of torque is set to 10, then torque increases 10 Kgfcm per second. The greater the increase rate of torque is, the shorter the fastening period of time is. If the increase rate of torque is too great, then the fastening torque overshoots, thus exceeding the target fastening torque (B).
FIG. 7 shows the waveform of the rotational speed of the bit 3 and torque to be used in the screwing apparatus shown in FIG. 5 based on the instruction as to the amplitude of electric current issued by the CPU 4. That is, an actual rotational speed (f) of the bit 3 and torque (d) can be obtained according to the instruction as to the amplitude of the electric current issued by the CPU 4.
The motor torque can be controlled when the screw head 7 is moving downward in the vicinity of the workpiece 8 even if the motor torque has an error of approximately 20%, but the rotational speed of the bit 3 cannot be appropriately controlled. When the bit 3 rotates at a high speed in the screwing region (a), the seating surface 7a of the screw head 7 collides with the workpiece 8. As a result, seating torque (d1), shown in FIG. 7, which is an impact torque generated when the rotation of the bit 3 has stopped, rises rapidly, thus exceeding torque (e), predetermined in the CPU 4, for deciding whether or not the screw 10 has been seated on the workpiece 8 and the target fastening torque (B). That is, the fastening operation terminates without fastening the screw 10 at the target fastening torque (B). That is, the torque (d1) in seating depends on the rotational speed of the bit 3 detected before the screw 10 is seated on the workpiece 8 and the amplitude of electric current (torque) flowing through the motor detected when the screw 10 is seated on the workpiece 8 and cannot be controlled stably by merely the control over the electric current flowing through the motor and the voltage to be applied thereto.